It is known practice to dye keratin fibers, for example, human keratin fibers such as the hair, with dye compositions comprising oxidation dye precursors, which are generally known as oxidation bases, such as ortho- or para-phenylenediamines, ortho- or para-aminophenols, and heterocyclic compounds such as diaminopyrazole derivatives. These oxidation bases are colorless or weakly colored compounds, which, when combined with oxidizing products, may give rise to colored compounds via a process of oxidative condensation.
It is also known that the shades obtained with these oxidation bases can be varied by combining them with couplers or coloration modifiers, the latter being chosen, for example, from aromatic meta-diamines, meta-aminophenols, meta-diphenols, and heterocyclic compounds such as indole compounds.
The variety of molecules used as oxidation bases and couplers allows a wide range of colors to be obtained.
The “permanent” coloration obtained by means of these oxidation dyes ideally satisfies at least one of a number of various characteristics. For example, the dye ideally has no toxicological drawbacks, allows shades to be obtained in the desired intensity, shows good fastness with respect to external agents such as light, bad weather, washing, permanent waving, perspiration, and/or rubbing, allows grey hair to be covered, and is as unselective as possible, i.e., produces the smallest possible color differences along the same keratin fiber, which is generally differently sensitized (i.e., damaged) between its end and its root.
It is also known practice to dye keratin fibers, for example, human keratin fibers such as the hair, with dye compositions comprising direct dyes. These dyes are colored and coloring molecules with an affinity for keratin fibers. They may be applied to the keratin fibers for a time necessary to obtain a desired coloration, and then may be rinsed out.
The standard direct dyes used include, for example, nitrobenzene dyes, anthraquinone dyes, nitropyridine dyes, azo dyes, cationic azo dyes, xanthene dyes, acridine dyes, azine dyes, triarylmethane dyes, and natural dyes.
The use of direct dyes is common since they have certain advantages over oxidation dye precursors, for example, reduction of the potential risks of allergy, absence of sensitization of the hair due to the oxidative process, and/or shorter leave-on times. In addition, the colorations obtained with direct dyes may be chromatic.
However, the colorations obtained with direct dyes may be temporary or semi-permanent, since the nature of the interactions that bind the direct dyes to the keratin fiber, and their desorption from the surface and/or the core of the fiber, are responsible for their low dyeing power and their poor fastness with respect to washing, bad weather, and/or perspiration. These direct dyes may also be light-sensitive due to the poor resistance of the chromophore to photochemical attack, which may lead over time to fading of the coloration of the hair.
Moreover, direct dye compositions are colored and, in oxidation dyeing, the coloration develops both in the composition and on the hair. Consequently, direct dyeing and oxidation dyeing have the drawback of making the application soiling.
Compounds of styryl and imine type existing in a colored form and in a colorless form and their use in the field of cosmetics, for example, in the field of skincare and nail varnishes, are described, for example, in Japanese Patent Nos. 56-150 006, 56-081 522, 56-025 106, 55-113 710, and 55-031 057.